In the metallization step utilized preparatory to etching of conductors about the outer surface of an integrated circuit, step-coverage of conductive metal films (typically aluminum alloys) is poor over surface discontinuities, such as recesses and contact holes/vias. Planarization of the conductive surface is particularly desirable when vias are stacked vertically in multilevel metallization.
Step coverage of film deposited conventionally by evaporation or sputtering becomes progressively worse as the dimensions of components on the integrated circuit shrink. The poor step coverage is a result of the shadow effect in the deposited film at the sidewalls of steps or holes.
The poor step coverage problem can be solved by selective chemical vapor deposition of tungsten, by metal deposition using high temperature and/or bias sputtering, or by supplemental metallic deposition, using multiple alternation sequences involving a combination of evaporation and resputtering. However, the surfaces resulting from these procedures are not planar.
The use of a pulsed laser to melt and planarize Al thin films to fill high aspect ratio contact vias is a very attractive approach to Ultra Large Scale Integrated (ULSI) circuit metallization. Laser planarization is a low thermal budget, simple, and effective technique for planarizing metal layers and filling interlevel contacts at the cost of only one additional step to the standard process flow. Excimer Laser planarization relies on a very short laser pulse to rapidly melt an absorbing metal layer. During the molten period, mass transport of the conductive metal occurs, which results in flow of the metal into contact holes/vias and drives the surface flat due to the high surface tension and low viscosity of molten metals.
Recently, the technique of laser planarization has shown promise in improving the step coverage of aluminum alloy films in micron/submicron geometry contacts and contact vias. However, due to the high reflectivity of aluminum (approximately 93% for wave lengths in the region down to 200 mm) and its relatively low evaporation temperature (2467.degree. C.), aluminum alloys suffer from the following disadvantages: (1) inefficient use of laser energy, (2) low optical ablation limit, and (3) low process window between the ablation limit and the via-fill limit.
Planarization systems utilizing excimer laser irradiation show particular promise for filling submicron-diameter contact holes/vias and planarizing the resulting surface. Lessening of the surface reflectivity normally encountered in heating of aluminum alloys by laser energy has already been reported as widening the "process window" between the "ablation limit", or temperature at which the conductive metal boils or evaporates, and the "via-fill limit," or temperature at which sufficient flow of the conductive metal occurs to fill the circuit recesses.
A general discussion of laser planarization can be found in a paper titled "Interconnects on Integrated Circuits Improved by Excimer Laser Planarization for Multilevel Metallization" by Mukai, et al., pp. 101-107, i.e., VLSI Multilevel Interconnection Conference, Santa Clara, CA (1988), which is hereby incorporated into this disclosure by reference. It describes the use of a thin copper overcoating to enhance aluminum planarization processing by increasing the initial optical absorbance of the laser beam in the conductive metal film. The paper fails to address the low oxidation resistance and the recognized difficulty of etching copper coatings.
Use of titanium as an anti-reflective coating for laser planarization processes has also been proposed. However, reported improvements in planarization were achieved at the expense of several drawbacks, including high resistivity and stress. The higher resistivity of the Ti--Al alloys that result from this process diminishes the advantage of Al metallization over an alternative metallization scheme using chemical vapor deposited tungsten as the primary conductive medium. Moreover, the higher stresses in the Ti--Al alloys imposes reliability concerns such as adhesion, cracks and stress voiding. It has been therefore concluded that titanium is not a desirable anti-reflective coating for aluminum and aluminum alloys in metallization procedures.
Despite the shortcomings in presently-reported systems for laser planarization, the value of an anti-reflective coating in widening the process window has been concluded to be important and to have demonstrated usefulness in increasing the thickness of a layer of conductive metal across a step or via.
A search for alternative anti-reflective coatings has led to identification of molybdenum as a useful coating. Molybdenum film is proposed as an anti-reflective coating on aluminum alloys or other low boiling point metals used for metallization purposes. The addition of a molybdenum film prior to laser planarization results in more efficient use of laser energy, less ablation of the aluminum layer at a given optical fluence, and widening of the process window.